1. Field of the Invention
The present invention relates to the introduction into a subsurface formation of a ferrous compound (understood herein to include any compound or complex containing one or more ferrous ions), as a reductant, to reduce a contaminant contained in the subsurface formation to a harmless species. In particular, the present invention is applicable to the treatment of subsurface formations containing hexavalent chromium and other contaminants, to convert the hexavalent chromium to trivalent chromium and the other contaminants to harmless forms.
2. Related Art
One example of hexavalent chromium contamination is at the Macalloy Corporation Superfund site in North Charleston, S.C. This is a former ferrochrome production facility where a large hexavalent chromium source area and several large dissolved phase hexavalent chromium plumes are present.
Methods for treating subsurface formations with reductants include processes forming zerovalent iron (iron fillings) based, permeable reactive barriers and in situ injection processes involving various chemical reductants (e.g., sodium hydrosulfite, also referred to as sodium dithionite) only. Zerovalent iron based reactive barriers are effective (at least in the short term) but are expensive to install and allow little flexibility with respect to installation depth and location. Zerovalent iron systems are very expensive to install at depths of greater than 25 ft. In addition, zerovalent iron based reactive barriers are not easily modified once installed. Chemical reductant technology involving, for example, injection of sodium hydrosulfite only converts existing iron (if present) in the aquifer to ferrous iron forms and thereby creates a redox zone. The success of injecting hydrosulfite only is very much dependent on the presence of sufficient reducible iron in the subsurface and the treatment longevity of injection of hydrosulfite only is limited by the amount of reducible iron present in the aquifer. In addition, normally a pH buffer must be added to the hydrosulfite solution to ensure that the hydrosulfite effectively reduces any reducible iron that might be present.
It was initially assumed that sodium dithionite would likely prove to be the best reductant for use at the site based on its successful use at other Cr(VI) impacted sites. However, laboratory study indicated that sodium dithionite had little effect on treatment of the Cr(VI) in the lake fill sediments at the Macalloy Corporation Superfund site.
Ferrous sulfate and ferrous chloride, however, were observed to very effectively treat Cr(VI) as evidenced by the inability to detect Cr(VI) in water and phosphate solution extracts from fill sediments following treatment. The addition of ferrous iron salts likely serves to provide a direct source of available (reduced) iron to promote conversion of Cr(VI) to the trivalent form. Although effective in the laboratory, a major concern with the use of ferrous salts such as ferrous sulfate or ferrous chloride in the field, relative to a reductant such as sodium dithionite, is the tendency for ferrous iron to rapidly precipitate out of solution during injection. Rapid precipitation can lead to well clogging and/or aquifer formation clogging. This poses a particular concern at the Macalloy Corporation site where the fill sediments/ground water are characterized by a high pH and a high pH buffering capacity. Such conditions would be expected to promote very rapid precipitation of iron. Observations of ferrous sulfate and ferrous chloride solutions in the laboratory showed that upon exposure to the atmosphere, ferrous iron begins to precipitate out of the solution almost immediately.
Other chemical reductants (e.g., other sulfur based reductants) are less powerful than sodium hydrosulfite and were found to be either ineffective or too costly in treating contaminants associated with the Macalloy site aquifer solids.